Generally, human eyes will sense white color as white color under any illuminating condition (for example, solar beam, fluorescent lamp, incandescent lamp). In the case of white color, the relative gains for red color (to be called hereinafter "R"), green color (to be called hereinafter "G"), and blue color (to be called hereinafter "B") are the same for each.
However, MID (MOS image devices), CCD (charge coupled devices) and image sensing tubes such as vidicon or plumvicon show different relative gains for R,G,B when the color rays pass through the respective filters under different illuminating conditions. The relative gains for R,G,B have a nature as shown in FIG. 1. In comparison with the condition of standard illumination (curve "a"), if the color temperature is low, R will have a high gain and B will have a low gain as shown in curve "b", while, if the color temperature is high, B will have a high gain and r will have a low gain as shown in curve "C". Accordingly, a white color image signal from a video camera will be stained with blue color if the color temperature of the illumination source is high, while the white signal will be stained with red color if the color temperature of the illuminating source is low, such being the disadvantage of the conventional device.
In an attempt to give a solution to this problem, there has been proposed an apparatus which can regenerate proper color signals by correcting the variations of the color temperatures through electrically controlling the gains of the R and B signal amplifiers. The controlling of the amplifiers is carried out in such a manner that the color having a low gain is amplified by increasing the gain, while the color having a high gain is amplified by decreasing the gain, so that, in the case of white color, the relative gains for R,G,B should be made equal to one another under any illuminating condition, thereby adjusting the tone of color.
However, if the R and B signal gains are too much different from the gain of the reference color G, then the electrical correction, i.e., the amplifying rate, has to be carried out to a great extent. If the amplifying rate is made very large, noise is also amplified, and therefore, the overall image quality is aggravated.
Therefore, a different device has been proposed in which the transmittance characteristics are optically adjusted for different wave lengths by means of an optical color temperature displacing filter. However, in this device, the characteristics of the filter. i.e., the gains for R, G, B, are decided during the manufacturing of the filter, and therefore, the automatic linear white balance correction is impossible, unlike the case of the electrical correction, with the result that the displacing filter has to be used as an auxiliary means for the electrical correction.
For the reason set forth above, there is the disadvantage that the operator of the video camera has to experience difficulties in achieving a proper translation of the color temperature filter. If the white balance is to be achieved without carrying out the displacement of the color temperature filter properly, the electrical correction will be excessively required, with the result that a degradation of the image signals will appear.
Therefore, an improved technique for adjustment of the white balance is disclosed in Japanese patent publication No. Sho63-31293. In this technique, an iris, a fluorescent lamp correcting filter and a color temperature adjusting filter which are movable back and forth are provided in such a manner that the color temperature and the incident beam amount should be adjusted in accordance with the illuminating conditions for the illuminated object. However, this last device is very complicated due to the requirement for an optical filter driver and an iris driver, and therefore, there is the disadvantage that the manufacturing cost of the device becomes very high.